The early 20th century saw the development of the first methods for catalytically producing sulfuric acid from sulfur dioxide. These developments gave rise to the first “contact” plants in which highly concentrated (98-99%) sulfuric acid was produced. In a first step, SO2 is oxidised to SO3 at very high temperatures (300-500° C.) and in a second step SO3 is converted into H2SO4 by addition of water at lower temperatures. These “contact” plants have changed only slightly in their manner of operation over the course of recent decades. One improvement was inter alia the introduction of the double contact plant which may in principle be regarded as two series-connected (single) contact plants. As a result, efficiency and thus conversion of SO2 to H2SO4 have been increased to above 99%.
EP 2 404 654 A1 describes, for example, such a method in which SO2 is converted in two stages into H2SO4 at high temperature (of 300° C. and 700° C.) and at high pressure (between 2 bar and 50 bar). Metal oxides are specifically stated as preferred catalysts.
Further plants which are the current state of the art are described for example in the document “Stand der Technik in der Schwefelsäureerzeugung im Hinblick auf die IPPC-Richtlinie” by Herbert Wiesenberger and Joachim Kircher, Umweltbundesamt GmbH, Vienna, 2001 or in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, volume A 25, pages 635 to 700.
The essential problems of these contact plants are:                1. The temperature differences for SO2/SO3 and SO3/H2SO4 conversions and the associated emission limit exceedances when starting up the plant.        2. General corrosion of the plants due to the temperatures and H2SO4 concentrations involved and the associated poor reliability and rapid ageing of contact plants.        3. High capital costs in conjunction with high operating costs for catalyst replacement and repairs.        4. Contact plants require minimum SO2 inlet concentrations of above 3 vol. %, below which H2SO4 conversion does not proceed.        5. Due to the nature of the system, the reactors consist of stainless steel because of the high catalysis temperatures and have a relatively large space requirement.        
One parallel invention of interest was the SULFACID® method dating from the 1960s. In this method, SO2 is oxidised to SO3 on an activated carbon bed at ambient temperatures and converted on the same activated carbon bed into H2SO4 by simultaneous addition of water. A dilute acid (10-20%) can be produced in this manner. This is a wet method which is, however, only capable of converting SO2 to H2SO4 to a limited extent. The SO2 input concentration is of economic interest only up to approx. 2500 ppm, since the activated carbon layer thicknesses otherwise increase hugely and result in a large pressure drop. Moreover, as a consequence, the efficiency of a conventional SULFACID® plant remains relatively low (approx. 90-95%). Furthermore, the space requirement of such plants rises massively, as a result of which conventional Sulfacid® plants are disadvantageous from the standpoint of capital and operating costs. This fact becomes all the more marked, the larger are the plants in terms of volumetric flow rate.
In a number of methods, the catalytic removal of sulfur dioxide from waste gases is carried out in a reactor charged with activated carbon, wherein the catalyst is repeatedly washed with water or a hydrous solution and then redried.
The catalyst is exposed to SO2 until the conversion of SO2 drops, then the SO2 feed is shut off and the catalyst exposed to water and the H2SO4 is washed out. The catalyst is then dried and may then be reused.
Such methods are known, for example, from U.S. Pat. No. 3,486,852; U.S. Pat. No. 4,122,150; U.S. Pat. No. 5,679,238 or U.S. Pat. No. 2012/251404 A1.
The disadvantage of these latter methods is that the catalyst must be repeatedly washed with water and then redried. A continuous method is thus not possible and at least two reactors must thus always be operated alternately.
Furthermore, only slightly concentrated sulfuric acid can be obtained by the above-stated method which must then either be dumped or alternatively highly concentrated in a costly manner before being offered for commercial sale.